Plasma is a state of matter, where matter particles can be free flowing in an ionic form under high-energy conditions. Plasma state can be artificially initiated by discharging a large amount of electric energy confined in a small space.
Several industrial applications that require localized application of high energy utilize plasma. For example, arc discharge in aqueous electrolytes, such as for welding under seawater, is widely used in engineering and construction.
Given the substantial amount of energy that can be utilized with plasma, and the level of spatial control of such application, the number of industrial applications of using plasma that can be contemplated is substantial. However, at present the only known form of stationary plasma discharge in liquid media is the arc discharge in liquid water.
In recent years, electric arc discharge in water has been used in several physico-chemical studies and in the synthesis of various materials. The specific feature of arc discharge in liquid media is the localization of a plasma region near the electrode ends and a “falling” form of volt-ampere characteristic.
The prior art offers several examples of attempts to generate plasma within a liquid. In the US there are a number of patents and published patent applications that describe methods and apparatuses for the initiation of plasma discharges within a liquid phase, where gas, phase-like bubbles are present, and for the use of this discharge for stimulation of chemical processes such as the decomposition of compounds and cracking of materials, which may be used in detoxification, for example.
In U.S. Pat. No. 7,067,204 to Nomura et al. (2006), “Submerged plasma generator, method of generating plasma in liquid and method of decomposing toxic substance with plasma in liquid” is proposed. A method and apparatus for generating plasma in a liquid is described. The apparatus includes an ultrasonic wave generator for generating bubbles in the liquid, and an electromagnetic wave generator for continuously irradiating electromagnetic waves into the liquid from within the liquid in order to generate plasma. The method of generating plasma in a liquid includes the steps of generating bubbles in the liquid by irradiating ultrasonic waves in the liquid, and generating plasma in the bubbles by continuously irradiating electromagnetic waves from within the liquid to the bubbles. This invention comprises various methods for generating the bubbles inside the liquid medium, such as a heating device, a decompression device or an ultrasonic wave generator. The gas-liquid ratio achieved by the latter described bubble generating method is small. Basically, the liquid phase prevails in the medium. Therefore, the steady burning zone of the discharge is quite small, resulting in a very small field of applications for the device.
In U.S. Pat. No. 5,270,515 to Long and Raymond (1993), “Microwave plasma detoxification reactor and process for hazardous wastes” is proposed. A large volume microwave plasma process for “in-situ” detoxification of dioxins, furans and other toxicants is disclosed. A helical coil and a cylinder of low loss dielectric tubing are coaxially positioned inside a microwave resonant cavity to extend from a cross-polarized fluid inlet to a cross-polarized vapor outlet. Fluid passing through the coil cylinder is directly ionized to the plasma state by microwave energy introduced into the cavity. The geometry of the coil relative to the cylinder induces a magnetic field in the plasma compressing the plasma to the center of the cylinder, thereby preventing charring of the cylinder walls. Said geometry also provides a slower fall through rate for the treatment of liquid and solid waste. The process and apparatus are particularly suitable for mobile applications, for on-site treatment of hazardous wastes. In the latter apparatus, a liquid medium is treated by the microwave irradiation for ionization. However, the latter method requires complicated equipment and high energy microwave irradiation and can be applied only for a restricted range of liquids. In the latter invention microwaves are used to ionize the fluid passing through the coil cylinder for producing plasma, which consumes large amounts of energy.
In U.S. Pat. No. 4,886,001 to Chang et al. “Method and apparatus for plasma pyrolyisis of liquid waste”, a method and apparatus for pyrolytically decomposing waste material is disclosed. The method is characterized by injecting a mixture of waste and water into a plasma torch having an operation temperature over 5000° C. to form a mixture of product gases and solid particulate. The gases and particulate are separated in a cyclone separator. A second cyclone separator and a partial vacuum separate any carryover gases from the particulate. The carryover gases and the particulate are treated in a scrubber with a caustic solution and water in order to eliminate any carryover particulate from the gases, and to neutralize hydrochloric acid (HCl) present in the gases. Finally the gases are removed from the scrubber. In the latter apparatus, plasma is only used as a high temperature source, used for decomposition of substances.
In US Patent Application 2004/0265137 A1, December/2004, and U.S. Pat. No. 7,384,619, “Method for generating hydrogen from water or steam in plasma”, both to Bar-Gadda a method is proposed for hydrogen production from water or steam by means of plasma discharge excited in the Ultra-high radio frequency or low radio frequency, as well as with arc discharge. Bar-Gadda describes the injection of water molecules into plasma discharge in the form of steam, which is usually produced by water boilers, therefore reducing the efficiency of the overall process.
In U.S. Pat. No. 7,070,634 B1 A1, April/2006, “Plasma reformer for hydrogen production from water and fuel” to Wang describes a plasma apparatus for converting a gaseous mixture of water vapor and hydrocarbons into hydrogen. The reaction chamber consists of an outer wall that acts as an emitter electrode and an inner wall that acts as a collector electrode. The mixture of water vapor and hydrocarbons are introduced between these two layers, inside a non-equilibrium thermal plasma environment. A non-combustion pyrolysis process is used to create this environment.
In Japanese Application JP2006273707 by Shibata et al., relates to the publication “Synthesis of amorphous carbon nanoparticles and carbon-encapsulated metal nanoparticles in liquid benzene by an electric plasma discharge in ultrasonic cavitation field”, Ultrasonic Sonochemistry 13 (2006) 6-12, Institute of Multidisciplinary Research for Advanced Material (IMRAM), Tohoku University. This application illustrates a method and a device for producing a nanocarbon material, which does not require an expensive production facility such as the ones normally required for dry treatment. It can easily produce the nanocarbon material because the application of high voltage is not needed, neither worsening nor deteriorating the working environment in a production premise, and at the same time considering the safety. This method can remarkably reduce production costs by improving production efficiency because of its continuous production and recovery, and providing an alternative for mass productivity. The method comprises a process (A) for arranging electrodes, one cathode and one anode, connected to a power source; an ultrasonic horn connected to an ultrasonic generator within an organic solvent that fills a container; and a process (B) for generating an ultrasonic cavitation field by ultrasonic waves into the organic solvent, around the head of the ultrasonic horn; and effecting the thermal decomposition of the molecules in the organic solvent by applying a voltage to the electrodes so as to generate plasma discharge within the ultrasonic cavitation field adequate for the production of the nanocarbon material.
U.S. Pat. No. 6,835,523, by Yamasaki et al., describes a “Method for fabricating with ultrasonic vibration a carbon coating”, which is a process for fabricating a carbon coating in a medium disposed on one side of an electrode connected to a high-frequency power supply. Ultrasonic vibrations are then supplied to the object.
Prior art (cited above) lacks control of a continuous operation, and require large amounts of power, which can be industrially infeasible. Therefore, given the substantial expected benefits of using plasma to induce chemical reactions to both break down compounds and synthesize new ones, there's a need for methods and apparatuses that provide continuous operation at a cost-effective level and modularity for scalable industrial applications.